1. Field of the Invention
The present invention relates to a laser Doppler velocimeter for emitting a laser beam on an object to be measured to measure a velocity and/or a displacement of the object due to shift of a frequency of the beam, i.e., Doppler shift and, more particularly, to a measuring apparatus for causing one measuring unit to simultaneously measure a two- or three-dimensional velocity vector.
2. Description of the Prior Art
Conventionally, when a two- or three-dimensional velocity vector is measured, two or three one-dimensional velocimeters are combined, and outputs from the velocimeters are calculated to obtain the velocity vector. In this case, the size and cost of each apparatus are undesirably increased in proportion to the number of dimensions, and high-precision setting of a measuring probe must be performed upon every measuring operations. For example, a multi-dimensional velocimeter using an optical fiber and a plurality of one-dimensional velocimeters is disclosed in Japanese Patent Provisional Publication No. 93258/1982. In such multi-dimensional measurement, however, the following problems are posed in practice.
In order to increase a resolution of a portion to be measured, beams from the probes in the velocimeters are focused to form a spot. Since a surface which scatters beams is a target as an object to be measured in many cases, scattered beams from a given measuring probe are focused into stray beams at another probe, thus generating crosstalk between the probes. Since this crosstalk cannot be separated from an original signal, a velocity cannot be measured in practice. This phenomenon will be described hereinafter with reference to FIG. 1.
FIG. 1 shows an arrangement upon measurement of a two-dimensional velocity. Referring to FIG. 1, a scattered particle 5 which is an object to be measured has a velocity vector V. The velocity vector V is divided into an X-direction velocity vector V.sub.x and a y-direction velocity vector V.sub.y. When laser beams having angular frequencies .omega..sub.1 and .omega..sub.2 are emitted from two measuring probes 1 and 2 which form an angle +.alpha. with respect to the vector V.sub.y, respectively, Doppler angular frequency shift components .DELTA..omega..sub.x and .DELTA..omega..sub.y are obtained as follows: EQU .DELTA..omega..sub.x =2.multidot.K.multidot.V.sub.x .multidot.sin.alpha. (1) EQU .DELTA..omega..sub.y =2.multidot.K.multidot.V.sub.y .multidot.cos.alpha. (2)
where K is the absolute value of a wave vector of each laser beam, and V.sub.x and V.sub.y are the absolute values of the velocity vectors respectively. A Doppler shift component .DELTA..omega..sub.151 of a signal beam which passes through a path of 1.fwdarw.5.fwdarw.1 and a Doppler shift component .DELTA..omega..sub.251 of a crosstalk beam which passes through a path of 2.fwdarw.5.fwdarw.1 are defined as follows: EQU .DELTA..omega..sub.151 =.DELTA..omega..sub.x -.DELTA..omega..sub.y ( 3) EQU .DELTA..omega..sub.251 =(.DELTA..sub.2 -.DELTA..sub.1)-.DELTA..omega..sub.y ( 4)
When independent laser beams are used as a light source for two axes, a difference between optical frequencies .omega..sub.2 -.omega..sub.1 of the two laser beams is added to a crosstalk component. In general, since the difference between optical frequencies .omega..sub.2 -.omega..sub.1 of two individual laser beams randomly and largely changes with an elapse of time, the angular frequency .DELTA..omega..sub.251 of the crosstalk component irregularly changes. Even if .omega..sub.2 =.omega..sub.1 can be realized, a relationship between the magnitudes of the two Doppler shift components .DELTA..omega..sub.151 and .DELTA..omega..sub.251 is changed in accordance with the directions of the velocity vectors, and only the original signal component .DELTA..omega..sub.151 cannot be separated. For this reason, in practice, it is impossible to combine a plurality of one-dimensional laser velocimeters, each having a laser such as an He-Ne laser which can select only one frequency as a light source, and to perform multi-dimensional measurement.
In order to prevent this, two types of lasers having apparently different frequencies have been used. In this method, however, a very large-sized and high-cost laser such as an Ar-ion laser must be used, and a drawback that cost is increased in proportion to the number of axes cannot be eliminated.